1. Field of the Invention
The present invention relates to an image forming apparatus, such as a printer, a facsimile machine, or a copier.
2. Description of the Related Art
In an electrophotography image forming apparatus, a charging device uniformly charges a surface of a photoconductor, an exposure device exposes the surface of the photoconductor based on image information to form a latent image, and a developing device develops the latent image by attaching toner in developer to the latent image. A toner image formed on the surface of the photoconductor by the developing is transferred to a recording material directly or via an intermediate transfer medium, and the toner image transferred to the recording material is fixed to the recording material by a fixing device. A cleaning device removes, from the surface of the photoconductor, residual toner remaining on the surface of the photoconductor after the transfer.
In an image forming apparatus described in Japanese Laid-open Patent Publication No. 2014-77975, a corona charger is used as the charging device. The corona charger generates a corona discharge by a strong electric field between a corona discharge electrode to which a high voltage is applied and a grid electrode facing the corona discharge electrode. The surface of the photoconductor is uniformly charged by using charges generated by the corona discharge.
An electrostatic dust collection action acts on the surface of a discharging member, such as the corona discharge electrode, so that toner floating inside the image forming apparatus is likely to adhere to the surface of the discharging member. Therefore, with use of the charging device over a long time, toner is accumulated on the surface of the discharging member. Consequently, a discharge distribution becomes uneven and a local discharge or the like may occur, resulting in a non-uniform discharge.
In the image forming apparatus described in Japanese Laid-open Patent Publication No. 2014-77975, an airflow is generated inside the image forming apparatus by an exhaust fan such that the airflow passes through the charging device in order to discharge scattered toner from the charging device and reduce toner attached to the surface of the discharging member. Therefore, it is possible to prevent toner from being accumulated on the surface of the discharging member with time, and to prevent occurrence of a non-uniform discharge.
Most of the toner removed from the surface of the photoconductor by the cleaning device is collected by the cleaning device. However, some of the toner is not collected by the cleaning device, and floats, as scattered toner, inside the image forming apparatus.
When a toner image, such as a photograph image, with a high image area ratio is formed on the surface of the photoconductor, the amount of toner attached to the surface of the photoconductor increases, and the amount of toner that is scattered without being collected from the surface of the photoconductor by the cleaning device tends to increase accordingly. Therefore, if the scattered toner is not efficiently discharged from the cleaning device by the airflow, a large amount of toner adheres to the surface of the discharging member and a non-uniform discharge is likely to occur, so that a non-uniform charge occurs on the surface of the photoconductor. This causes image density unevenness resulting in an abnormal image.
Therefore, if the flow velocity of the airflow is increased by rotating the exhaust fan at a high speed by supposing a case in which a large amount of toner is scattered, the scattered toner can efficiently be discharged from the charging device by the airflow, and the amount of toner attached to the surface of the discharging member can be reduced.
However, if the flow velocity of the airflow is increased by rotating the exhaust fan always at a high speed, even when a toner image, such as a photograph image, with a high image area ratio is not formed on the surface of the photoconductor and the amount of scattered toner is small, the exhaust fan is rotated at a higher speed than needed. Therefore, an extra noise, such as the sound of the rotating exhaust fan or the sound of the discharged airflow, may be generated, for example.
Even in a charging device of a roller charging system, which charges the surface of the photoconductor by a discharge of a charging roller serving as the discharging member, the same issues as described above occur.
In view of the above, there is a need to provide an image forming apparatus capable of preventing occurrence of an extra noise and preventing an abnormal image from being formed due to a non-uniform discharge.